IEEE P802.22 Wireless Rans s9
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July 2006 doc.: IEEE 802.22-06/0131r0
IEEE P802.22 Wireless RANs
Explicit Out-band Signaling (EOS) for Hidden Incumbent System Detection (with supplements)
Date: 2006-07-14
Author(s): Name Company Address Phone email Samsung Advanced Tae-In Hyon Institute of Korea +82-31-280-9503 [email protected] Technology Sangjo Yoo Inha University Korea +82-32-860-8304 [email protected]
Abstract Explicit out-band signaling is used to overcome the situation of there being a hidden incumbent system that could cause the CPE’s in the WRAN cell edge areas to be out of synchronization with the BS, thus causing the overall system to incur interference to the incumbent system. This (July) edition edition supplements the document “SAIT-MAC(0222).doc” written in March, 2006, by adding the reinforcement of the hidden incumbent reporting scheme with acknowledgements, improving the probability of coexistence among multiple WRAN BSs for outband signaling, and efficient channel usage of the explicit outband signaling by utilizing the fractional bandwidth usage method.
Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.22.
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures
Submission page 1 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0
16.4 Explicit Out-band signaling for hidden incumbent system detection
16.4.1 Hidden incumbent scenarios
In the WRAN system, the WRAN BS may not be able to detect or be informed of the existence of an incumbent signal. This situation may arise when the incumbent system signal does not reach the WRAN BS so that the BS cannot sense it. And CPEs that are inside the coverage areas of both the WRAN BS and incumbent system cannot report this overlapping to the WRAN BS because they are not able to decode the WRAN DS signal due to the strong interference. This very serious case is called the “hidden incumbent system case”.
Figure 1 shows the typical scenario for a hidden incumbent system. For WRAN service, assume WRAN BS had sensed some channels and it recognized channel F was available (it may use a geographical channel usage database information for that area). When an incumbent system begins service with the same channel F, the BS does not know the existence of incumbent system because the BS is located outside radio transmission range of incumbent system. Therefore, as shown in Figure 1, WRAN CPEs that are located in overlapped area of WRAN and incumbent system are not able to decode the WRAN DS signal because of strong incumbent system interference. In this case, BS will keep its service on channel F and it will cause strong interference to incumbent system users possibly for a long time.
Radio range of TV station
Ch F TV BS
Radio range of Ch F 802.22 BS
WRAN BS : Not overlapped CR CPE : Overlapped CR CPE (can’t decode CR Signal and can’t report anything) : Not overlapped CR CPE
Figure 1 - Example of a hidden incumbent system
This hidden incumbent system case may occur not only during the WRAN BS initialization period (service start time) but also during any service period. Incumbent systems can start their service at any time without any notification or may initiate their service disregarding the service pattern as stored in WRAN database. Therefore, WRAN BS can mistakenly change its service channel to the channel that that is already being used by an incumbent system because of its unreliable sensing results.
The hidden incumbent system case causes the following problems: i) harmful interference to the incumbent system users ii) some WRAN CPEs cannot communicate with WRAN BS
An implicit incumbent system detection method as described in Section 21 can be used to resolve this hidden incumbent system case. The implicit detection method is based on the periodic sensing report exchanges between BS and CPEs after normal association procedure. But if some CPEs suddenly turn off their power without any notifications, then WRAN BS is not able to distinguish between the hidden incumbent system interfering case and
Submission page 2 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0 power off case. And sometimes, CPEs in the overlapped area may not have associated with the WRAN BS yet, in which case the BS cannot be expected to recognize any problems associated with an incumbent system being detected.
In this section, to resolve the hidden incumbent system case, an explicit out-band signaling method is used. Through some extra channels that are not currently used, a short message is broadcast. The implicit method of Section 21 can be used together with the explicit method as a supplementary algorithm.
16.4.2 Explicit Out-band signaling for hidden incumbent case detection
To address the hidden incumbent system case, BS periodically broadcasts out-band signal that includes some information about WRAN BS’s current channel through some unoccupied channels (e.g., candidate channels) that are not used for BS service channel or incumbent system. This broadcast out-band signal follows the same PHY and MAC frame architecture (to not necessitate additional protocol or PHY modulation). In addition, the SCH includes a flag (see Table 1) to indicate whether the current MAC frame is for regular service or out-band broadcast signal. Explicit out-band signal DS-Burst includes service channel information, such as current BS’s service channels and candidate channels. When some CPEs are not able to decode the BS’s current service channel, they will try to sense other channels to locate the BS signal. If CPEs receive the explicit out-band broadcast signal, the CPEs will know what the current service channel ID is. If the current channel was already sensed and was determined not to be decodable at the CPEs, then the CPE will send a report to the BS using the out-band upstream resource. After receiving the report, the BS changes its service channel to another one that is unoccupied because BS notices the existence of the hidden incumbent in the current band.
Figure 2 shows the explicit out-band signaling for hidden incumbent case detection. Channel F is used by the BS for its service. To solve the problem that is caused by the hidden incumbent system, the BS broadcasts out-band signal periodically by in the available channels B, C and D. If the CPE can not decode the signals of channel F, the CPE starts sensing other channels for detecting out-band signal from the BS. When the CPE detects the out- band broadcast signal of the BS in channel C, it transmits hidden incumbent system report message (see Table 5) to BS with upstream resource that exists in out-band broadcast signal.
Incumbents Incumbents
Outband Outband Outband Signal Signal Signal broadcast broadcastbroadcast WRAN Ch A Ch E Ch C Ch D Ch F BS Ch B
N-4 N-3 N-2 N-1 N N+1 N+2 N+3 N+4 N+5 Frequency
Report service signal Incumbents Incumbents Outband Outband Outband Not Signal Signal Signal available broadcast broadcastbroadcast Ch A Ch E CPE Ch B Ch C Ch D Ch F
N-4 N-3 N-2 N-1 N N+1 N+2 N+3 N+4 N+5 Frequency Figure 2 - Example of explicit out-band signaling
16.4.2.1 BS Out-band signal broadcasting
The BS sets the EOS field value in the SCH periodically and broadcasts the out-band signal MAC frame in channel B, channel C and channel D which is selected among candidate channel list as shown in Figure 3. The EOS field indicates whether the transmitting MAC frame is a general MAC frame or out-band signal MAC frame. Submission page 3 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0
The CPEs can detect the signal of BS using to the defined explicit out-band signaling period because the BS broadcasts out-band signal in other available channels, none of which are currently being used. The explicit out- band signaling period is decided by the BS. If a BS receives Hidden Incumbent System Report messages from CPE during the predefined period, it will recognize that current service channel F can not be decoded by the CPE(s). So, the BS must move the service channel to another channel. This process is illustrated in Figure 5-1.
channel Outband Signal Ch D broadcast Ch C CPE that cannot decode the The CPE finally receives at current service channel will least one of the periodic try to sense other channels outband broadcast signals Ch B
Time
Explicit outband signaling period
It can be decided by incumbent system detection time requirement. Figure 3 - Explicit out-band signaling
Although the basic scheme is that the out-band signal is broadcast in candidate channel B, channel C and channel D at the same time, the BS may choose to broadcast out-band signal sequentially in time for simplification of system organization, as shown Figure 4.
Broadcasting out-band signals at the same time like Figure 3 requires multiple PHY transceivers at the BS. On the other hand, the sequential out-band signal transmission as described in Figure 4 only requires one transceiver for out-band signaling. But, this sequential method has a drawback that the probability witha which the CPEs can detect the out-band signal decreases.
Outband Signal channel broadcast Ch D CPE that cannot decode the Ch D current service channel will Ch C try to sense other channels Ch C Ch B Ch B
Time
Explicit outband signaling period The CPE finally receives at least one of the periodic It can be decided by incumbent outband broadcast signals system detection time requirement. Figure 4 - Sequential explicit out-band signaling
Basically, out-band signal does not include the CPE list that should respond to the out-band signal with Hidden Incumbent System Report because the BS does not know which CPEs are located in the overlapped area in advance. But, optionally BS may choose to include a CPE list to poll specific CPEs. However, in this case the polled CPE should respond even if the current service channel is available to the CPE.
16.4.2.2 CPE hidden incumbent reporting
If the CPE can not decode the signal of the BS, it will detect out-band signal after sensing other channel as shown in Figure 5-2. If a signal in the current service channel is not decodable by the CPE because of incumbent system’s interference, then the CPE sends “Hidden Incumbent System Report” (see Table 5) to BS using US- Burst in the out-band. Hidden Incumbent System Report can indicate either “the current service channel F is not
Submission page 4 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0 decodable” or “the current service channel F is used by an incumbent system” if the CPE can recognize incumbent signal. Also, the report can include sensing results for some other channels.
start start
EOS = 1(in FCH) Sensing the other Select Out-bands in channel candidate channel list
Detect Out-band Broadcast signal Out-band signal
Is Incumbent NO Start Timer service detected in Done current channal?
YES Receive Hidden Send Hidden Timeout Incumbent Incumbent System Report System Report
Move to another Done Move to another channel channel Figure 5-1 BS Figure 5-2 CPE
To reduce reporting overhead and possible report collisions, only CPEs that are not able to decode the current service channel because of strong interference from other system (e.g., incumbent system) respond with “Hidden Incumbent System Report,” except when the CPE is explicitly polled by the BS.
Because the BS does not know which CPEs will send the report in advance, BS generally cannot allocate US resource to each CPE that will send the report. Therefore collisions of reports may be occurred. To increase the probability of successful report delivery to the BS, during one explicit outband signaling time k (2 ~ 3) number of MAC (including DS and US) frames are transmitted as shown in Figure 6. In SCH of DS the information for k should be included. For the first US1 all CPEs that detected hidden incumbent systems will try to send reports. As in Figure 7, in DS-2 and DS-3 BS gives acknowledgements for the previous reports to indicate successful report receptions. So that during the US-2 and US-3, only CPEs that did not receive acknowledgements for the previous reports will try sending reports again.
Submission page 5 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0
Explicit outband K (2 or 3) frame time signaling time Frame time
DSK US DS1 US1 K time Explicit outband signaling period
MAC Slot Number r
e US-Burst #1 US-Burst #2 b
m FCH u US-Burst #3 N
l a n P n A a H M DS-Burst h - C P
C . . .
S S A C U M A - M S
l D a c i g o
L US-Burst #n-1 US-Burst #n
Downstream Upstream Figure 6 – Explicit outband signaling time.
DS-1 US-1 DS-2 US-2 DS-3 US-3 A reports C reports K
Only CPEs that could not receive ACK for the first reports try to send second reports
Figure 7 – Report and Acknowledgment (k=3 case)
16.4.2.3 CPE upstream bandwidth allocation for hidden incumbent report
For the CPE Hidden Incumbent System Report message, there needs to be a process or method for allocation of upstream resource to CPEs. To reduce this overhead, the BS can allocate upstream resource for CPE’s Hidden Incumbent System Report (see Table 5) by using one of the following two methods.
i) First method is that the BS divides US resource into US-Burst slots for all unknown CPEs. When the BS transmits Out-band signal frame, it divides upstream resource according to maximum hidden incumbent report size and allocates each upstream burst. Also, it transmits US-MAP including each upstream burst's allocation information. When a CPE sends the Hidden Incumbent System Report, the CPE randomly selects a US-Burst, and then the CPE sends the Hidden Incumbent Report with the selected US-Burst. Figure 8 shows out-band signal MAC frame of slotted method.
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MAC Slot Number r
e US-Burst #1 US-Burst #2 b
m FCH u US-Burst #3 N
l a n P n A a H
M DS-Burst h - C P
C . . .
S S A C U M A - M S
l D a c i g o
L US-Burst #n-1 US-Burst #n
Downstream Upstream Figure 8 – Out-band signal MAC frame of slotted method
ii) The second method is to use the CDMA code. The PHY supports the usage of a CDMA mechanism for the purpose of hidden incumbent system report.
As specified in the PHY spec, the PHY has a subset of the available CDMA codes that shall be used for contention-based CDMA. The CPE, upon needing to transmit a hidden incumbent system report, shall select a CDMA code with equal probability from the code subset allocated for hidden incumbent notification. The CDMA code subchannel shall be selected by the PHY amongst the ones reserved by the MAC for hidden incumbent report.
When the BS transmits out-band signal, it allocates resource to transmit the CDMA code in upstream burst. A CPE transmits the selected random CDMA code in order to be assigned upstream resource to transmit the Hidden Incumbent System Report message. The BS shall provide upstream allocation for the CPE. But instead of indicating a basic CID, the broadcast CID shall be sent in combination with a CDMA_Allocation_IE, which specifies the transmit region and code that were used by the CPE. This allows a CPE to determine whether it has been given an allocation by matching these parameters with the parameters it used. The CPE shall use the allocation to transmit the Hidden Incumbent System Report. In addition, if allowed by the BS, data could also be transmitted in this allocation. Figure 9 shows Out-band signal MAC frame of CDMA method.
If the BS does not issue the CDMA_Allocation_IE, the CPE shall assume that the Code transmission resulted in a collision and try Random Backoff.
MAC Slot Number r e b
m FCH u N
l e a d n o P n c A
a H M h DS-Burst A US-Burst - C P C M
S S A D C U
M . . . C A - M S
l D a c i g o L
Downstream Upstream Figure 9 - Out-band signal MAC frame of CDMA method
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16.4.2.4 Co-existence between WRAN BSs for Out-band signaling
Transmitting multiple out-band signals has a possibility to collide with out-band signals of other co-located WRAN BSs. If communication between BSs is possible, the BSs can coexist by sending out-band signal in candidate channels at different time after exchanging broadcasting time schedule with each other. But if communication between BSs for exchanging their time schedule is not available, to avoid out-band signal collision, BSs randomly select their out-band signal broadcasting time within the pre-defined explicit out-band signaling period as shown in Figure 10 and Figure 11. Figure 10 illustrates the case when BS broadcasts outband signaling messages at multiple channels simultaneously. And Figure 11 is for the case that BS broadcasts outband signaling message sequentially in time at the different channels.
To guarantee that CPE decodes the outband signal within a short time interval without sensing time discordance, CPEs should try to find the signal at one channel at least during one explicit outband signaling period.
channels BS1 BS2 BS2 CH C CH C CH C CH C
CH B CH B CH B CH B
CH A CH A CH A CH A time To avoid collisions between BSs, Explicit outband signaling period within the outband signaling period the broadcating time of each BS is randomly decided. Figure 10 – BS coexistence for outband signaling.
To avoid collisions between BSs, BSs within the sequence of channels is randomly selected.
BS1 CH B CH C CH A CH C CH B CH A
BS2 CH A CH B CH C CH C CH A CHB
Figure 11 – BS coexistence for outband signaling (sequential case)
16.4.2.5 Efficient channel use for Explicit Outband Signaling with Fractional Bandwidth Usage method
Explicit Outband Signaling does not need a full bandwidth of 6 MHz channel. Broadcasting some channel information through an explicit out-band signal message requires only small bandwidth as shown figure 12. WRAN PHY defines “OFDMA with Fractional Bandwidth Usage” (see 8.1.2.3.4) that allows WRAN BSs to use a fractional bandwidth of full 6MHz. WRAN BS broadcasts Explicit Outband Signals with only BWEOS bandwidth at the beginning part of 6MHz. If a BS that is broadcasting Explicit Outband Signal detects that other BS uses the remained part, then it will try to change the channel for Explicit Outband Signaling. Figure 13-1 and figure 13-2 illustrate how explicit outband signaling BS and other CR WRAN BS for data transmission operate.
Submission page 8 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0
BWEOS Other CR system can use it. EOS
frequency Channel X 6 MHz
Figures 12 – Efficient channel use for Explicit outband signaling with Fractional bandwidth usage.
Find out available channel
Broadcast Explicit Outband Signal periodically with BWEOS MHz on channel X Yes Full empty No channel available ?
No On channel X , other system signal detected Use it. Yes Only BWEOS MHz No (or reported )? is used for EOS by other WRAN BS? Yes
Yes No use the remained ( 6MHz-BWEOS) Incumbent system? Other CR WRAN BS uses for data transmission for data transmission (hole band or fractional band )
No EOS band (BWEOS) is now clear out? Stop sending EOS on Find out other available channel X channel Yes
Find out other available Use the full bandwidth No channel Empty channel available?
Yes No Empty channel available? Stop sending EOS on channel X Yes
Change channel X for EOS to a new empty channel
Figure 13-1 BS Figure 13-2 other WRAN BS
Submission page 9 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0
Table 1 – Superframe control header format Syntax Size Notes Superframe_Control_Header_Format() 1 OFDM symbol long and transmitted with well-known { modulation/coding (e.g., QPSK rate ½) ST = 0 7 bits System Type Indicates the type of the system using this band. CT 1 bits Content Type Indicates the type of the content following the transmission of the SCH. Superframe = 0 CBP Beacon = 1
FS 8 bits Frames per Superframe Indicates the number of frames within a superframe. Frames within a superframe have a fixed size which preferably does not change across superframes. FDC 8 bits Frame Duration Code TTQP 16 bits Time To Quiet Period The amount of time it will take for the next scheduled quiet period. This serves to synchronize the quiet period of overlapping BSs. the TTQP is divided into two subfields: Time Scale and Time. The Time subfield consists of a 15 bit unsigned integer number. DQP 16 bits Duration of Quiet Period The estimated duration of the next scheduled quiet period. This is specified similar to the TTQP field. PP 1 bit Preamble Present Indicates whether the preamble of the following frame is present or not. For example, a frame preamble may not be needed when the cell is operating in a single physical (i.e., TV) channel EOS 1bit Indicates whether the current MAC frame is for regular service or for out-band broadcast signal. Tx ID 48 bits Address that uniquely identifies the transmitter of the SCH (CPE or BS) CN 8 bits Channel Number Indicates the starting physical (i.e., TV) channel number used by the BS. NC 5 bits Number of Channels In case channel bonding is used, this field indicates the number of additional consecutive physical (i.e., TV) channels used by the BS. In the basic mode, NC = 3 (i.e., three TV channels). Reserved 1 bit GIF 1 bit Guard Interval Factor Specifies the GIF used by the PHY in the frame transmissions of this superframe. Pre-determined values are: 0 = O-QAM 4 = Default mode used for superframe transmission Length 8 bits The length of the information following the SCH Submission page 10 Tae-In Hyon, Samsung July 2006 doc.: IEEE 802.22-06/0131r0
IEs Variable Optional Information Elements which would be transmitted after the SCH. They are: MAC version Current transmit power Part 74 acknowledgement Location configuration information HCS 8 bits Header Check Sequence }
Table 2 – Bulk Measurement Report (BLM-REP) Message format Syntax Size Notes BLM-REP_Message_Format() { Management Message Type = 41 8 bits Transaction ID 16 bits Number of Single Measurement Reports 8 bits The number of single measure ment reports contained in this message Single Measurement Reports Variabl Table 3 e }
Table 3 – Single Measurement Report Message format Syntax Size Notes Single_Measurement_Report_Format() { Element ID 8 bits Table 4 Length 8 bits Transaction ID 16 bits Report Information Element Variabl Table 4 e }
Table 4 – Report Information Elements Element ID Length Description (1 byte) (bytes) 129 Variable TV Measurement Report 130 Variable Part 74 Measurement Report 131 Variable Beacon (802.22 Related) Measurement Report 132 Variable CPE Statistics Measurement Report 133 Variable Location Configuration Measurement Report 134 Variable Beacon (Part 74) Measurement Report 133 Variable Consolidated Spectrum Occupancy Measurement Report 132 Variable Channel Feedback Measurement Report # (ex. 135) Variable Hidden Incumbent System Report – Table 5 131-255 Reserved
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Table 5 – Hidden Incumbent System Report Message format Syntax Size Notes Hidden_Incumbent_System_Report_Format() { Report Mode 4 bits 6 Start Frame 8 bits Frame number in which the channel measurement start ed Duration 16 bits The actual duration of the measurement
Channel Number 8 bits Number of Channels 8 bits for (i = 1; i Number of Channels; i++) { Channel State 3 bits Table 7 } }
Table 6 – Report mode Syntax Size Notes Report_Mode_Format() { Late 1 bit Indicates whether this CPE is unable to carry out a measurement request because it received the request after the requested measurement time. The Late bit shall be set equal to 1 to indicate the request was too late. The Late bit shall be set to 0 to indicate the request was received in time for the measurement to be executed, or if no start time was specified. Incapable 1 bit Indicates whether this CPE is incapable of generating this report requested by the BS. The Incapable bit shall be set to 1 to indicate the CPE is incapable. The Incapable bit shall be set to 0 to indicate the CPE is capable or the report is autonomous. Refused 1 bit Indicates whether this CPE is refusing to generate this report requested by the BS. The Refused bit shall be set to 1 to indicate the CPE is refusing. The Refused bit shall be set to 0 to indicate the CPE is not refusing or the report is autonomous. Unmeasured 1 bit CPE did not measure the channel }
Table 7 – Channel state information Value State 000 Unmeasured 001 Vacant 010 Occupied 011 Occupied by Incumbent 100 Occupied by 802.22 101-111 Reserved
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